This invention relates generally to the field of controls for synchronous motors and, more particularly, to a method and apparatus for stabilizing the rotational alignment of the synchronized rotor of a synchronous motor.
Some applications of hysteresis synchronous motors require extremely precise rotational alignment or angular positioning of the rotor during operation. For example, in high energy particle experiments, an intense pulsed neutron source is generated by bombarding a target which emits a stream of neutrons. The neutrons are then passed through a "chopper", which is a cylinder having aligned slits of predetermined width that by accurate timing can yield the speed of flight of the neutrons and thereby determine their energy. The chopper is rotated by a hysteresis synchronous motor at approximately 270 revolutions per second and the slits of the chopper allow neutrons to pass only when they are properly angularly aligned during rotation. The highly precise and uniform rotation required can be affected by many factors such as intermittent bearing drag, changes in wind currents, or fluctuations in the drive frequency or current.
A hysteresis synchronous motor in its normal operating mode is synchronized with the drive frequency such that the rotor lags the stator magnetic axis by a stable load angle determined by the load on the motor. An intermittent bearing drag or other fluctuation can cause additional lag of the rotor such that the rotor magnetic axis would lag the stator magnetic axis by the steady state load angle plus an angular increment. The larger the increment, the larger the magnetic force on the rotor, causing it to tend to return to its steady state load angle after the intermittent fluctuation in load is gone. This force tends to cause the rotor to overshoot, causing the load angle to decrease beyond the steady state load angle. The rotor thus "hunts" to get back to its desired angular alignment, resulting in undesirable oscillations which can last many seconds if not damped. Another source of oscillations can occur when the drive frequency is changed by small increments for the purpose of changing the speed of the motor. When such a small change occurs, a load angle increment builds up until the rotor is pulled toward alignment, at which time it will overshoot again, resulting in phase oscillations about the steady state load angle. These problems are particularly acute for large inertia loads and they severely limit the ability to maintain stable and precise angular alignment that is required in many applications. Thus, it is highly desirable to be able to damp out these oscillations in an economical manner without having to physically modify the basic design of the motor itself.
In the prior art, a number of methods for controlling motors have been developed. A substantial number of methods have been developed for generating and controlling the current that is supplied to the armature of the motor to control the velocity of the motor, but not the phase of the rotor. These methods are unsuitable for hysteresis synchronous motors which do not have armature windings or currents. Further, these methods do not provide the needed precise phase control of the rotor. Another prior art system synchronizes the rotor position with an external reference, but does not provide damping or phase stabilization of the rotor. Many controllers are simply designed to develop the proper current to drive the motor. None of the known prior art motor control systems provide active damping of rotor oscillations about its normal synchronous phase for a hysteresis synchronous motor.
It is accordingly an object of this invention to provide an improved method and apparatus for maintaining rotational alignment of the rotor of a synchronous motor.
It is another object of the invention to provide an improved method and apparatus for damping phase oscillations of the rotor of a hysteresis synchronous motor caused by minor load changes and small abrupt changes in drive frequency.
Still another object of the invention is to provide damping of excursions from rotational alignment of the rotor of a hysteresis synchronous motor by phase shifting the stator magnetic axis in response to an error signal proportional to a change in the rotor period of rotation.
Yet another object of the invention is to provide apparatus for minimizing rotational alignment oscillations of a rotor in response to abrupt changes in drive frequency by phase shifting the input power with phase shift amplifiers to start the rotor towards realignment earlier than would otherwise occur.